Compacted graphite is the name usually given to flake graphite which has become rounded, thickened and shortened as compared to normal elongated flakes commonly found in gray cast iron. This modified form of graphite has also been known by various other names, such as "vermicular", "quasi-flake", "aggregate flake", "chunky", "stubby", "up-grade", "semi-nodular" and "floccular" graphite.
Most cast irons have elongated flake graphite structures and such irons are comparatively weak and brittle, but have good thermal conductivity and resistance to thermal shock. It is also possible to produce cast irons having a nodular graphite structure and these are ductile and comparatively strong, but they have lower thermal conductivity and in some instances poorer resistance to thermal shock than gray iron. Advantageously, irons with compacted graphite structures combine the high strength and ductility of nodular graphite irons with good thermal conductivity and resistance to thermal shock evidenced by gray iron.
U.S. Pat. No. 4,036,641 discloses a method for treating molten carbon-containing iron to produce a cast iron with compacted graphite structure comprising adding to the molten iron in a single step an alloy containing silicon, magnesium, titanium and a rare earth, the balance being iron. The alloy contains a minimum of 3 percent magnesium and the ratio of titanium to magnesium is in the range of 1:1 to 2:1.
U.S. Pat. No. 4,086,086 is directed to an improvement in the alloy and method of U.S. Pat. No. 4,036,641 in that there is included in the alloy 2 to 10 percent of calcium. The presence of this element is said to produce compacted graphite cast irons with a wider range of initial sulfur contents.
For some years the "inmold" process has been used successfully for production of ductile iron. In such process untreated molten gray iron is introduced into the mold cavity by way of a conventional pouring system which additionally includes one or more intermediate chambers containing a nodularizing agent in an amount sufficient to convert the graphite to nodular or spheroidal form.
British Pat. No. 1,559,168 relates to a modification of such inmold process wherein, instead of the product being nodular or spheroidal graphite iron castings, the product is cast iron with compacted graphite. The agent for providing the iron with compacted graphite is a 5 percent magnesium ferrosilicon alloy containing cerium. Such agent or alloy may, in addition to containing 5 percent magnesium, contain 0.3 to 0.5 percent calcium, 0.2 percent cerium, 45 to 50 percent silicon and balance iron. Titanium may be added separately to the metal in the ladle before being cast or included in the alloy. The patent also sets forth process parameters, including the base area of the intermediate chamber, to obtain a given magnesium content in the cast metal.
European patent application No. 0 067 500, published Dec. 22, 1982, is directed to inmold treatment of molten iron to produce on a relatively consistant basis castings containing 30 to 70 percent nodular graphite and balance compacted graphite. The addition may comprise a free-flowing combination of about 6 percent magnesium and balance ferrosilicon (50 percent). The addition may also be in the form of preforms of agglomerated particles, cast solid preforms, or particles suspended in a resinous binder. The addition does not include titanium except in noneffective trace amounts, since this "deleterious" element is said to inhibit nodularity.
European patent application No. 0 020 819 published Jan. 7, 1981 is directed to a process for making compacted graphite cast iron using an addition having a fine sieve analysis (1-3 mm particles). The composition of the addition is not given. Rather the application indicates that the composition of the addition is known and comprises silicon, magnesium, titanium, calcium and rare earth metals. The addition is believed to be that of U.S. Pat. No. 4,036,641 (supra).
Since about 1976, Foote Mineral Company, Exton, Pa., has sold alloys designed for producing compacted graphite iron. Although such alloys vary somewhat in composition, they all contain on the order of at least about 2.8 magnesium, with some containing 4.5 to 5.5 percent magnesium, and a maximum of about 10 percent titanium. In such alloys the ratio of titanium to magnesium is quite low not exceeding about 3.6:1, and for several of the alloys the ratio is on the order of 1.3:1 to 2.5:1, depending on the particular alloy. In advertising literature pertaining to these commercially available alloys, one alloy containing 2.8 to 3.3 percent magnesium and 8 to 10 percent titanium, and having a Ti/Mg ratio of about 3:1, is indicated as having utility in the inmold process.
Rather extensive tests of various of these prior known alloys have failed to result in the production of compacted graphite iron when used in the inmold process. On occasion compacted graphite iron was obtained in parts of castings or in a mold, but this type of iron could not be consistently obtained over a wide range of conditions. Thus, such alloys are inadequate for use in the inmold process.